1. Technical Field
The present invention is directed to control circuitry for controlling the output of a signal power amplifier. More specifically, the present invention is directed to feedback control circuitry for controlling the output level of a signal power amplifier to one of a plurality of output levels.
2. Background Information
Generally, signal power amplifiers typically used in radio apparatus are operated at a specific output power level, and such power amplifiers are adjusted to that specific output power level during the manufacturing process of the specific signal power amplifier. Additionally, applications exist which require signal power amplifiers to have the capability of outputting one of a plurality of output power levels, as, for example, in mobile telephone systems. Specifically, in mobile telephone systems, eight discreet power levels are typically required.
Generally, mobile telephone systems employ a multichannel access system having communication channels between a base station and mobile units consisting of control channels and voice channels. The mobile unit, when initiating a call, generates a call-initiation signal and a dial-signal, representing the called party's telephone number, which gets transmitted over the control channel. Upon receipt by the base station, the base station transmits a response signal, acknowledging the calling party, and a channel signal, designating to the calling party which voice channel to employ. In this way, a communication link between the mobile unit and the called party is established.
When the mobile unit is in transit, the received radio frequency (RF) signal level from the mobile unit to the base station is proportional to the distance between the mobile unit and base station. In order to prevent channel interference, it is preferable that the received RF signal level be constant. For this reason, the base station transmits a level control signal to the mobile unit for use by the mobile unit in adjusting its output RF signal power in accordance therewith. Specifically, the level control signal varies the RF signal power output level to one of eight different output power levels.
Automatic power control circuitry for performing this function is well known in the art. For example, as shown in Japanese laid-open application No. 54-112,111 assigned to the NEC Corportation, and herein incorporated by reference, the output RF signal power level produced is determined by one of eight different reference voltages. These reference voltages are used by a differential amplifier for comparing the chosen reference voltage value to the feedback voltage of the output of the power amplifier. These reference voltages are applied selectively to the differential amplifier by an analogue multiplexer, the individual reference voltages being generated via resistance networks having variable resistors connected to the analogue multiplexer. The level control signal is used to select the appropriate reference voltage supplied to the differential amplifier by controlling the analogue multiplexer.
The prior art system of employing variable resistors for generating the plurality of voltage levels used by the RF amplifier to adjust its output power level has inherant limitations. For example, the variable resistors must be adjusted accurately during manufacture of the resistence networks and subsequent adjustments required due to age and component degradation is difficult. Additionally, should a reference voltage be required to be adjusted for other reasons, such adjustment would also be difficult. Further, the analogue multiplexer is both expensive and large, making any automatic power control circuitry employing analogue multiplexers both costly and large.